1. Field of the Invention
The invention is related to the field of opto-isolation circuits, and in particular, to opto-isolation circuits capable of autonomously outputting at least two different signals.
2. Statement of the Problem
Coriolis mass flow meters measure mass flow and other information with respect to materials flowing through a pipeline. These flow meters typically comprise a flow meter electronics portion and a flow meter sensor portion. Flow meter sensors have one or more flow tubes of a straight or curved configuration. Each flow tube configuration has a set of natural vibration modes, which may be of a simple bending, torsional, radial, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural vibration modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. When there is no material flowing through a Coriolis flow meter sensor, all points along the flow tubes oscillate with a substantially identical phase. As material flows through the flow tubes, Coriolis accelerations cause points along the flow tubes to have a different phase. The phase on the inlet side of the flow meter sensor lags the driver, while the phase on the outlet side of the flow meter sensor leads the driver.
FIG. 1 illustrates a Coriolis flow meter 5. Coriolis flow meter 5 comprises a Coriolis flow meter sensor 10 and Coriolis flow meter electronics 20. Flow meter electronics 20 is connected to flow meter sensor 10 via path 100 to provide for mass flow rate, density, volume flow rate, totalized mass flow information, and other information over path 26. Port 26 may output information, such as measurements generated by the flow meter 5.
The flow meter sensor 10 includes a pair of flanges 101 and 101′, manifold 102, and flow tubes 103A and 103B. Connected to flow tubes 103A and 103B are a driver 104, pick-off sensors 105 and 105′, and temperature sensor 107. Brace bars 106 and 106′ serve to define the axis W and W′ about which each flow tube 103A and 103B oscillates. Although a dual tube, curved meter is shown, it should be understood that the discussion herein will equally apply to a meter having a single tube or a meter having a straight tube or tubes.
When flow meter sensor 10 is inserted into a pipeline system (not shown), the material of the pipeline enters the flow meter sensor 10 through the flange 101, passes through the manifold 102, where the material is directed to enter the flow tubes 103A and 103B, flows through the flow tubes 103A and 103B and back into the manifold 102 from where it exits the flow meter sensor 10.
The flow tubes 103A and 103B are selected and appropriately mounted to the manifold 102 to have substantially the same mass distribution, moments of inertia, and elastic modules about the bending axes W-W and W′-W′, respectively. The flow tubes 103A-103B extend outwardly from the manifold 102 in an essentially parallel fashion.
The flow tubes 103A-103B are driven by the driver 104 in opposite directions about their respective bending axes W and W′ at what is termed the first out of phase bending mode of the flow meter 5. The driver 104 may comprise any one of many well known arrangements, such as a magnet mounted to flow tube 103A and an opposing coil mounted to flow tube 103B and through which an alternating current is passed for vibrating both flow tubes. A suitable drive signal is applied by the flow meter electronics 20, via lead 110, to the driver 104.
The pick-off sensors 105 and 105′ are affixed to at least one of flow tubes 103A and 103B on opposing ends of the flow tube to measure the oscillation of the flow tubes. As the flow tubes 103A-103B vibrate, the pick-off sensors 105-105′ generate a first pick-off signal and a second pick-off signal. The first and second pick-off signals are applied to the leads 111 and 111′.
The temperature sensor 107 is affixed to at least one of the flow tubes 103A and 103B. The temperature sensor 107 measures the temperature of the flow tube in order to modify equations for the temperature of the system. The path 112 carries temperature signals from the temperature sensor 107 to the flow meter electronics 20.
The flow meter electronics 20 receive the first and second pick-off signals appearing on the leads 111 and 111′, respectively. The flow meter electronics 20 process the first and second pick-off signals to compute the mass flow rate, the density, and/or other properties of the material passing through the flow meter sensor 10. This computed information is applied by the meter electronics 20 over the path 26, such as to an external device or devices.
FIG. 2 shows a typical prior art output circuit that can be used to generate a flow meter output in one of two communication formats. The figure includes two optocouplers, where the optocouplers comprise an electrical isolation between the meter electronics 20 and the output port 26, for example. This may be done to limit electrical power consumption, for example, wherein the meter electronics 20 (and/or the flow meter assembly 10) cannot draw electrical power beyond the capacity of the isolation device. This protects against damage in the event of an electrical short, for example. This may be done where the flow meter 5 is used in an explosive or hazardous environment. The isolation may be part of an intrinsic safety (IS) construction of the meter electronics 20, wherein the barrier may prevent excessive electrical power from being transferred across the barrier and between safe and hazardous areas.
In the figure, the upper optocoupler is used to transfer the signal from the input to the output. Because the signal may be converted into at least one other communication format, the output may include a conversion circuit. As a result, the output of the upper optocoupler can be selectively provided to the conversion circuit, dependent on a control signal.
The control signal is provided by the meter electronics 20. The control signal therefore can be the result of a command or data that is stored in memory in the meter electronics 20, wherein a processor or other circuitry sends the resulting control signal to the lower optocoupler. Alternatively, the control signal can be received from an external device and relayed to the lower optocoupler.
In the prior art, the lower optocoupler passes the control signal. The control signal is used to select the output format by selecting the conversion circuit. If the control signal does not select the conversion circuit, then the signal is outputted as-is (i.e., a “raw” signal). If the conversion circuit is selected, the conversion circuit converts the signal into a new format that is available at the output.
Opto-isolation may be used for IS applications where the meter electronics 20 and one or more external devices are in communication. Opto-isolation may be used for IS applications where the meter electronics 20 must communicate across a barrier between safe and hazardous areas. Unfortunately, suitable optocouplers are expensive and large. Further, each optocoupler consumes electrical power.
For the meter electronics 20 of FIG. 1, two ten millimeter optocouplers may be utilized to generate both the desired raw signal and control signal, as shown in FIG. 2. This is costly both in terms of component costs and board real estate, because such optocouplers are large and expensive components.
What is needed, therefore, is an improved opto-isolation circuit that does not require a separate control signal.